This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2001-231737, filed Jul. 31, 2001; and No. 2002-152981, filed May 27, 2002, the entire contents of both of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical fiber, an optical fiber component capable of efficiently inducing a non-linear phenomenon, and an optical transmission method.
2. Description of the Related Art
The quantity of communication information tends to exponentially increase with growth of the information society, and wavelength-division multiplexing transmission (WDM transmission) technology or the like has attracted attention with such increase in information. Wavelength-division multiplexing transmission is an optical transmission system suitable for mass high-speed communication since this is a system which transmits light having a plurality of wavelengths through one optical fiber.
The wavelength-division multiplexing transmission is currently carried out by applying an erbium doped optical fiber type optical amplifier (EDFA), and light which has a high output in a wavelength band used by the optical fiber can be obtained owing to advancement of this EDFA. A gain band of EDFA is typically a 1.55 xcexcm wavelength band. It is to be noted that the 1.55 xcexcm wavelength band indicates a wavelength band having the wavelength of approximately 1550 nm at the center, e.g., 1530 nm to 1570 nm.
There has been examined a technique which performs frequency conversion of signal light by utilizing a non-linear phenomenon generated in the optical fiber by using such light with a high output. This frequency conversion technique mainly utilizes a mixture of four light waves which is a non-linear phenomenon, and converts a wavelength of the signal light by causing the signal light whose wavelength is other than the zero-dispersion wavelength to enter the optical fiber having a function to induce the non-linear phenomenon while causing exciting light in the vicinity of the zero-dispersion wavelength of that optical fiber to enter the optical fiber. As the exciting light, light having a high output such as output light of the above-described EDFA is applied.
When the signal light having a longer wavelength than the zero-dispersion wavelength of the optical fiber is caused to enter the optical fiber while causing the exciting light to be incident on the optical fiber having a function to induce the non-linear phenomenon, it is converted into light having a shorter wavelength than the zero-dispersion wavelength. On the contrary, when the signal light having a shorter wavelength than the zero-dispersion wavelength of the optical fiber is caused to enter the optical fiber while causing the exciting light to be incident on the optical fiber having the function to induce the non-linear phenomenon, it is converted into light having a longer wavelength than the zero-dispersion wavelength.
The non-linear phenomenon which occurs in the optical fiber greatly depends on the intensity of the light entering the optical fiber, the spot size of light which is propagated in the optical fiber (effective core cross-sectional area), and the quantity of dopant such as GeO2 existing in the area where the light is propagated.
That is, the non-linear phenomenon which occurs in the optical fiber is apt to be generated as the intensity of the light entering the optical fiber is higher, as the spot size of the light propagated in the optical fiber is smaller and as the quantity of the dopant existing in the area where the light is propagated is larger.
In order to utilize the above-described non-linear phenomenon, an optical fiber having, e.g., such a single-peaked index profile as shown in FIG. 6 is applied in the prior art. This optical fiber includes a core 1 consisting of a quartz-based material obtained by doping GeO2 in the center and a cladding region 5 covering the circumference of the core 1, and the cladding region 5 is formed of pure quartz and has a uniform refractive index.
There has been conventionally examined for the frequency conversion application of the optical fiber having a small spot size of the light propagated in the optical fiber by increasing a concentration of the dopant (GeO2) doped in the core 1 of such an optical fiber having a single-peaked optical index profile and increasing a difference in specific refraction index between the core 1 and the cladding region 5.
In the prior art optical fiber having the single-peaked index profile, however, in the case of trying to obtain the optical fiber having, e.g., the zero-dispersion wavelength in the wavelength 1.55 xcexcm band which is the amplification band of EDFA, an effective cutoff wavelength of the optical fiber exceeds the 1.55 xcexcm wavelength band when a difference in specific refractive index between the core and the cladding region is set equal to or larger than 2.3% by increasing the concentration of GeO2 as shown in FIG. 7, and the single mode cannot be obtained in the optical fiber, thereby disabling efficient induction of the non-linear phenomenon.
Therefore, as described in the excerpt C-3-15 of the 1998 meeting of IEICE (the Institute of Electronics, Information and Communication Engineers), the optical fiber which has been conventionally proposed for induction of the non-linear phenomenon has a limit in the mold field diameter up to 4.6 xcexcm and a difference in specific refractive index up to approximately 2.1%.
Further, as the wavelength band to be used enlarges, induction of the non-linear phenomenon on the wavelength side shorter than 1.55 xcexcm is demanded, but the single-mode fiber cannot be obtained with the wavelength shorter than 1.55 xcexcm in case of the above-described fiber. Therefore, a difference in the specific refractive index must be further lowered, but such a technique deteriorates the non-linearity of the fiber, which is not efficient.
Furthermore, when use of the four light wave mixing is attempted, the slope of the wavelength dispersion in the zero-dispersion wavelength and the wavelength band used is important, and the fiber with a smaller dispersion slope is demanded. As described in C-3-15 of the 1998 meeting of IEICE (the Institute of Electronics, Information and Communication Engineers), however, the dispersion slope is approximately 0.04 ps/nm2/km, and further reduction in the dispersion slope is demanded. In the simple single-peaked index profile, however, further reduction in the dispersion inclination is difficult, and the conversion wavelength band is difficult to enlarge.
It is an object of the present invention to provide an optical fiber capable of efficiently inducing the non-linear phenomenon.
It is another object of the present invention to provide an optical fiber component including an optical fiber capable of efficiently inducing the non-linear phenomenon.
It is a further object of the present invention to provide an optical transmission method using an optical fiber capable of efficiently inducing the non-linear phenomenon.
According to a first aspect of the present invention, there is provided an optical fiber comprising: a core; and a cladding region which covers the outer periphery of the core, having a zero-dispersion wavelength in a wavelength range of 1.4 xcexcm to 1.65 xcexcm, and being a single mode in the zero-dispersion wavelength, wherein GeO2 is doped in the core in a quantity such that a relative refractive index difference of the core becomes not less than 1.8%, the cladding region includes a first cladding region which covers the outer periphery of the core, a second cladding region which covers the outer periphery of the first cladding region, and a third cladding region which covers the outer periphery of the second cladding region, and a refractive index of the second cladding region is smaller than that of the first cladding region and smaller than that of the third cladding region.
According to a second aspect of the present invention, there is provided an optical fiber comprising: a core; and a cladding region which covers the outer periphery of the core, having a zero-dispersion wavelength in a wavelength range of 1.4 xcexcm to 1.65 xcexcm, and being a single mode in the zero-dispersion wavelength, wherein GeO2 is doped in the core in a quantity such that a relative refractive index difference of the core becomes not less than 1.8%, the cladding region includes a first cladding region which covers the outer periphery of the core and a second cladding region which covers the outer periphery of the first cladding region, and a refractive index of the first cladding region being smaller than that of the second cladding region.
According to a third aspect of the present invention, there is provided a quartz-based optical fiber which induces a non-linear phenomenon in the optical fiber, comprising: a core; and a cladding region which covers the outer periphery of the core, having a zero-dispersion wavelength in a wavelength range of 1.4 xcexcm to 1.65 xcexcm, and being a single mode in the zero-dispersion wavelength, wherein GeO2 is doped in the core, a relative refractive index difference of the core is not less than 1.0%, the cladding region includes a first cladding region which covers the outer periphery of the core, a second cladding region which covers the outer periphery of the first cladding region, and a third cladding region which covers the outer periphery of the second cladding region, the second cladding region is arranged in a range which influences light propagated in the core, and a refractive index of the second cladding region being smaller than those of the first cladding region and the third cladding region.
According to a fourth aspect of the present invention, there is provided a quartz-based optical fiber with a W-shaped index profile, comprising: a core; a cladding region which covers the outer periphery of the core, having a zero-dispersion wavelength in a wavelength range of 1.4 xcexcm to 1.65 xcexcm, being in a single mode in the zero-dispersion wavelength, and inducing a non-linear phenomenon in the optical fiber, wherein GeO2 is doped in the core, a relative refractive index difference of the core is not less than 1.0%, the cladding region includes first cladding region which covers the outer periphery of the core, a second cladding region which covers the outer periphery of the first cladding region and a third cladding region a third cladding region which covers the outer periphery of the second cladding region, the second cladding region is arranged in a range which influences light propagated in the core, and a refractive index of the first cladding region is smaller than that of the second cladding region.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.